Seventeen-year-old Andy and his visually impaired younger step-sister Piper are left devastated and parentless after the sudden death of their father. With no remaining relatives, child protective services places the siblings into the temporary foster care of Laura, a gentle former child counselor living in a secluded suburban home. Upon arriving, Andy notices that the house has an eerily neglected backyard pool filled with dark, stagnant water, and meets Oliver, a quiet pre-teen boy who has been completely mute since Laura took him in. Laura explains that her own daughter Cathy tragically drowned in the backyard pool two years prior, claiming that taking in foster children helps heal her shattered spirit. However, Andy quickly notices deeply disturbing occurrences. In the dead of night, Laura leads a trance-like Oliver into the locked basement, reciting rhythmic folk incantations. When Andy investigates, he discovers strange occult talismans, jars containing human hair and teeth, and historical journals detailing forbidden soul-transference rituals. Realizing that Laura is not trying to heal her foster children but use their physical bodies as living vessels to pull Cathy's spirit back from the dead, Andy tries to escape with Piper. But Laura cuts their communication lines, drugs Andy's food, and barricades the home. As the final phase of the ritual approaches under a torrential thunderstorm, Piper must rely on her sharp ears and sheer willpower to navigate the pitch-black house, rescue her poisoned brother, and confront a foster mother willing to sacrifice everyone to see her daughter again.

In the harrowing climax, Laura drags a semiconscious Andy and the mute boy Oliver out to the rain-drenched swimming pool. Laura forces Oliver into the deep end, chanting an invocation to summon Cathy's restless water spirit into the boy's body. The spectral entity violently surges through the water, possessing Oliver and twisting his limbs in a terrifying, unnatural display. But when Cathy's spirit takes control, she does not exhibit loving recognition. Instead, the entity is a vicious, vengeful water demon deformed by the agony of death. As Laura kneels weeping at the edge of the pool expecting a sweet maternal embrace, the possessed entity grabs Laura by the throat and drags her headfirst into the deep end, pulling her under into the dark depths. Piper, armed with a pool skimmer pole and following the splashing sounds through the storm, reaches the water and manages to pull a coughing, gasping Andy onto the concrete deck. The two siblings huddle together as police sirens illuminate the backyard. In the pool below, Laura's lifeless body sinks beside the discarded ritual artifacts, proving that grief, when weaponized against the living, only drags the mourner into the grave.